[Voyage-linux] Static IP on Alix

[Adrian Reyer]

Hi Roberto,

On Sat, Sep 03, 2011 at 12:00:47PM +0200, Roberto Arena wrote:
> * auto eth0
> iface eth0 inet static
>        address 192.168.1.127
>        netmask 255.255.255.0
>        broadcast 192.168.1.255
>        gateway 192.168.1.249
>        dns-nameservers 212.216.112.112*
> * auto eth1
> iface eth1 inet static
>        address 192.168.1.128
>        netmask 255.255.255.0
>        broadcast 192.168.1.255
>        gateway 192.168.1.249
>        dns-nameservers 212.216.112.112*

What does the '*' mean? If they actually are in the file it is wrong.
If the resolvconf package is installed it should work. At least it does
so here.
However, having 2 interfaces within the same IP range asks for trouble.
Why do you want to have eth0 and eth1 within the same network?
- playing switch
  -> use a bridge instead
- adding a second IP
  -> use eth0:0 for a virtual IP instead
- redundant links to the same network
  -> use a bridge on the 2 devices
- boosting performance
  -> use a bond-device on the 2 devices if your switch supports bonds
You can combine the above.

If you stick with your configuration and both network interfaces link to
the same physical network, e.g. no proxy arp involved, be aware, the
linux kernel always uses the interface for outgoing packets that comes
first in the routing table, no matter which interfaces IP address you
connect to.
You can get round this by using source routing with alternate routing
tables. That way you can make sure IP a traffic leaves the box via
interface a and IP b traffic leaves the box via interface b.

Another problem might be IP spoof protection. If you have traffic coming
in via an interface that has no route to that IP it will be dropped.
You can fix this by disabling spoof protection for that interface with
echo 0 > /proc/sys/net/ipv4/conf/eth0/rp_filter
or permanently by adding this to /etc/sysctl.conf.

The kernel answers a packet via the first interface that sees the
packet, no matter if the IP is right. If I recall correctly, this can be
changed with
echo 0 > /proc/ss/net/ipv4/conf/eth0/shared_media
or, again, by adding this to /etc/sysctl.conf
Obviously you need to state these lines for every interface in question.

Another point might be your switch. It might block your ports if it
detects packets with same MAC address on 2 interfaces the same time,
again as it might think your did a short circuit. Check if you can
configure your switch to suit your needs or if you need a bridged device
with STP enabled.

Regards,
	Adrian
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